For instance, JP2004-56855A (corresponding to US2004/0008002A1) teaches a motor drive system. Here, the contents of US2004/0008002A1 are entirely incorporated herein by reference. In this motor drive system, a feed back control operation (hereinafter referred to as an F/B control operation) is executed as follows. Specifically, a pulse signal of an encoder, which is provided in an electric motor, is counted up or down according to a rotational direction of a rotor of the motor. A rotational position of the rotor is sensed based on this encoder count value. The rotor is rotated by sequentially changing an exciting phase of the motor until the encoder count value reaches a target count value, which corresponds to a target rotational position of the rotor. In the above motor drive system, a failure of the FIB control system of the motor is monitored. When the failure of the F/B control system is sensed, the operation is changed to an open loop control operation. In the open loop control operation, the exciting phase of the motor is sequentially changed without relying on the feedback of the information of the encoder count value, and an exciting phase change count value is counted up or down according to the rotational direction of the rotor every time when the exciting phase of the motor is changed. In this way, the rotor is rotated until the exciting phase change count value reaches an open loop target count value, which corresponds to the target rotational position of the rotor. In this system, besides the encoder, an output shaft sensor, which senses a rotational angle (shift position) of an output shaft of the motor, is provided. The open loop control operation is executed based on the rotational angle (the shift position) of the output shaft of the motor, which is sensed with the output shaft sensor, upon the sensing of the failure of the F/B control system.
Lately, it has been demanded to reduce the number of the required components and the costs. In view of such a demand, it has been proposed to eliminate the output shaft sensor. However, in JP2004-56855A (US2004/0008002A1), the reference position for the open loop control operation (e.g., an initial position at the time of starting the open loop control operation) is set based on the output signal of the output shaft sensor. Thus, when the output shaft sensor is eliminated, the reference position for the open loop control operation cannot be set. Thus, the open loop control operation cannot be executed.